1. Field of the Invention
The present invention relates to a novel oxidation catalyst for the removal of carbon monoxide and hydrocarbon compounds from industrial gas streams.
2. Description of the Prior Art
Carbon monoxide, a partial combustion product of many fuels, is emitted by many sources such as gas turbine power plants, reciprocating engines, coal-fired boilers, heaters, and the like. Hydrocarbons are either concomitantly emitted with carbon monoxide, as incomplete combustion products of fuels, or generated by other industrial processes such as refinery dry gas and incinerator exit gas. Catalytic oxidation of these pollutants to carbon dioxide and water over a wide temperature range can achieve the continually decaying pollution limits set by law economically. In fact, catalysts have been used in the U.S. for automobile emissions control and gaseous emissions from industrial facilities. To date, the oxidation catalysts commercially used for these purposes are dominantly supported precious metals, such as platinum and palladium. The precious metal catalyst is superior to other catalysts in that it combines high activity and resistance to water vapor poisoning. However, the high cost of precious metals and their propensity to sulfur poisoning are drawbacks of this type of catalyst system. The high cost becomes a primary issue in processes where the metals cannot be recovered, for example, as with the platinum-containing carbon monoxide oxidation catalyst in the Fluid Catalytic Cracking (FCC) process--the most important unit in oil refineries. In the FCC process, carbon monoxide is formed in a regenerator during the combustion of the coke-containing catalyst from a riser reactor. Formation of carbon monoxide needs to be eliminated to prevent the regenerator and exit flue gas pipeline from the damage caused by the post-combustion of carbon monoxide, as well as to meet environmental regulations. A supported-precious metal is currently being used as the preferred carbon monoxide oxidation catalyst. Different from catalysts used in automobile catalytic converters, the carbon monoxide oxidation catalyst is injected into the FCC unit in particulate form, and is eventually carried away with the exit gas stream, and thus the catalyst cannot be recovered. The retention time of the catalyst in the FCC unit is significantly shorter than the life time of an automobile's catalytic converter. On the other hand, there is significant amount of sulfur dioxide formed in the regenerator. Sulfur dioxide poisons the oxidation catalyst usually by forming sulfates on the surface; however, the sulfates may be reduced in the reducing atmosphere when the catalyst is transferred to the riser reactor. Therefore, it would be of great advantage to substitute the expensive precious metal catalyst used in the FCC process with an inexpensive material.
Substitution of precious metals used in oxidation catalysts with ordinary base metals has long been of extensive interest in the catalysis field. For example, a layered catalyst comprising alumina with an inner layer of metal oxide and an outer layer of copper oxide is disclosed by Stephens et al., in U.S. Pat. No. 3,226,340. The use of copper chromite with other metal oxides for the treatment of exhaust gases from internal combustion engines is disclosed by Stiles, in U.S. Pat. No. 3,230,034. An active oxidation catalyst was prepared by depositing layers of oxides of copper, cerium and chromium with cerium oxide as the middle layer on an alumina support as described by Schenker et al., in U.S. Pat. No. 3,789,022. Since the 1970's, perovskite-type mixed oxides (ABO.sub.3) have attracted great attention as heterogeneous oxidation catalysts. Notably, R.sub.1-x Pb.sub.x MnO.sub.3 and RCoO.sub.3 (where R denotes the rare earth elements La, Pr, and Nd) compounds were intensively studied because they showed high promise to substitute platinum for CO oxidation. (See, for example, R.J.H. Voorhoeve et al., Science, Vol. 177,353-4, 1972 and Science, Vol. 195, 827-33, 1977). The Spinel oxide (AB.sub.2 O.sub.4) is another well-studied class of mixed oxides. For example, Hopcalite catalysts, based on manganese and copper oxides, have long been known as carbon monoxide oxidation catalysts. However, most of base-metal catalysts reported show inferior performance to the precious metal catalysts in the oxidation of CO in terms of catalytic activity, stability, and resistance towards poisoning by water and sulfur compounds. Recently, a carbon monoxide removing catalyst comprising an alumina support with deposited platinum and iron, cobalt, nickel, manganese, copper, chromium, tin, lead or cerium was reported (Japanese Patent 61 035 853).
Accordingly, it is an object of this invention to provide a highly active and stable oxidation catalyst, containing no precious metal components, for oxidation and removal of carbon monoxide and hydrocarbon compounds.